Selective catalytic reduction (SCR) is a known method for catalytically reducing nitrogen oxides (NOX) to N2 and water in high O2-content gas streams using a reductant material, such as ammonia (NH3). Such gas streams may be produced, for example, by diesel engines, some gasoline fueled engines and many hydrocarbon-fueled power plants as exhaust gases. The exhaust gases from such engines are typically passed through exhaust aftertreatment systems that contain a collection of catalyst materials designed to treat the exhaust gases. When the SCR method of exhaust gas treatment is employed, the reductant material is injected into the exhaust gas stream upstream of a suitable catalyst material. In the presence of the catalyst, an amount of NOX in the gas stream is reduced to N2 and water using the reductant. Selective catalytic reduction systems using NH3 as the reductant in exhaust aftertreatment systems are sometimes referred to as “NH3-SCR” systems.
Silicoaluminophosphates (SAPOs) are currently being studied for use as the catalyst material in SCR systems because of their selectivity in the reduction of NOX, opposed to the oxidation of NH3, in addition to their excellent hydrothermal durability. SAPOs are synthetic microporous crystalline materials having an aluminophosphate framework, with silicon atoms incorporated therein. The framework of SAPO-n consists of tetrahedral oxides of SiO2, AlO2− and PO2+, where n denotes a particular framework type. Negative framework charges occur within SAPO frameworks when there are more aluminum atoms than phosphorus atoms within the framework. These negative framework charges are typically balanced by H+ cations (i.e., positively charged ions) after the SAPO materials are synthesized and calcined; this form is usually referred to as H-SAPO-n.
To increase the selectivity and NOX reduction activity of SAPO-based catalyst materials, the non-metal charge balancing cations, such as H+, located within the SAPO frameworks may be replaced or exchanged, to some extent, by transition metal cations to form Me-SAPO, where Me denotes the cation-exchanged metal. Current ion-exchange methods involve immersing SAPO particles in a metal salt solution comprising cations of a transition metal and anions of a non-metal. In this method of ion-exchange, the SAPO particles are typically immersed in the metal salt solution for a relatively long period of time (e.g., 2-24 hours) with stirring. In addition, this immersion step may need to be repeated several times in order to obtain uniformly high ion-exchange levels throughout the SAPO frameworks. Accordingly, this liquid ion-exchange method typically produces a large amount of wastewater.
There is therefore a need for a more efficient method of replacing or exchanging charge balancing non-metal cations with transition metal cations in SAPO frameworks having negative framework charges.